Liquid metal cooled nuclear reactor constructions

ABSTRACT

The primary vessel of a pool type fast reactor is open at its upper end and is suspended from the roof of the vault by ties. The vessel is enclosed within a fluid tight leak jacket containing an inert cover gas for the pool of coolant. 
     Because of the unrestrained open end of the vessel severe stress concentrations in the upper strake are avoided. The problem does not occur in the leak jacket because the cover gas, supplemented by thermal insulation cladding and cooling coils, avoids excessive temperature differentials.

BACKGROUND OF THE INVENTION

This invention relates to liquid metal cooled nuclear reactorconstructions.

One known example of liquid metal cooled nuclear reactor constructioncomprises a primary vessel suspended from the roof of a containmentvault, the primary vessel containing a pool of coolant in which thereactor core is submerged. The primary vessel is sealed at its rim tothe roof of the vault and contains an inert cover gas for the pool ofcoolant. A leak jacket or catchpot enveloping the primary vessel is alsosuspended from the roof and serves to catch coolant in the event of aleak from the primary vessel.

When the reactor is in operation the temperature of the concrete at theunderside of the roof is approximately 40° C. whilst the temperature ofthe coolant in contact with the wall of the primary vessel isapproximately 400° C. so there is a large temperature differential overthe upper strakes of the primary vessel. This temperature differentialcreates a stress problem in the primary vessel and at its sealedconnection with the roof of the vault.

In U.S. Pat. No. 4,022,656 means for alleviating this stress problem isdisclosed. The means comprises a cooling jacket and thermal insulationcladding an upper region of the primary vessel and arrangedprogressively to reduce the temperature of the vessel wall towards theroof of the vault.

An object of the present invention is to modify the reactor constructionin such a manner that complex stresses in the wall of the primary vesselare largely avoided.

SUMMARY OF THE INVENTION

According to the invention in a liquid metal cooled nuclear reactorconstruction comprising a primary vessel suspended from the roof of aconcrete containment vault, the primary vessel containing a pool ofcoolant in which the reactor core is submerged, the primary vessel hasan open upper end and is suspended from the roof by ties, the primaryvessel and ties being enveloped by a fluid tight leak jacket which issealed to the roof of the vault at its rim and in which there is aninert cover gas for the pool of coolant. In a construction according tothe invention there is no circumferential restraint at the attachment ofthe primary vessel to the roof structure so hoop stresses at the rim areavoided. The leak jacket is not in contact with the coolant and beingcooled by cover gas in contact with practically all of its internalsurface, is not subject the severe temperature differential between thesurface of the pool and the roof of the vault. If desired acircumferential upper region of the leak jacket may be lined internallywith thermal insulation to reduce heat transfer by radiation from thepool of coolant and may be cooled externally. In a preferredconstruction the ties comprise rods or straps pivoted or welded to aninverted skirt attached externally to the primary vessel at a levelbelow the surface level of the pool of coolant.

DESCRIPTION OF THE DRAWINGS

Constructions of liquid metal cooled nuclear reactor embodying theinvention are described, by way of example only, with reference to theaccompanying drawings wherein:

FIG. 1 is a sectional view of a construction and

FIGS. 2, 3, 4 and 5 are fragmentary sectional views of alternativefeatures of the construction.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 there is shown a liquid metal cooled nuclear reactorconstruction comprising a primary vessel 1 suspended from the roof 2 ofa containment vault 3. The primary vessel 1 contains a pool 4 of coolant(having dynamic surfaces designated `S`) in which a reactor core 5 issubmerged. The primary vessel 1 has an open upper end and is suspendedfrom the roof 2 by ties 6 disposed about its periphery. The primaryvessel 1 and ties 6 are enveloped by a fluid tight leak jacket 7 orcatchpot which is sealed at its rim to the roof 2 of the vault. The leakjacket 7 contains an inert cover gas for the coolant. The upper regionsof the leak jacket are lined internally with thermal insulation 8 andthere is cooling means 9 comprising fluid conducting cooling coils onthe outer surface.

In more detail, the nuclear reactor core 5 is a fast breeder reactorcomprising a plurality of upstanding fuel element assemblies arrangedside-by-side on a diagrid 10. The core 5 is surrounded by a shroud 11and a pump 12 is arranged to flow coolant drawn from the outer regionsof the pool through the core by way of the diagrid. The coolant isheated in passage through the core and flows upwardly through the shroud11 thence outwardly to a heat exchanger 13 subsequently being returnedto the outer regions of the pool. The temperature of the coolant withinthe shroud is approximately 600° C. whilst the temperature of thecoolant outside the shroud is approximately 400° C.

There are fifty ties 6 equally spaced about the periphery of the primaryvessel 1 and, as shown in FIG. 2, each tie comprises a rod pivotablyconnected to the vessel. The upper ends of the ties are supported in theroof 2 of the containment vault in part spherical bearings 14. Thus theties 6 are freely pivotable at each end thereby avoiding complexstresses due to thermal expansion of the vessel and ties. Complex hoopstresses in the primary vessel are also avoided, because the rim of theprimary vessel is unrestrained. The inert cover gas is in contact withalmost all the inner surface of the leak jacket and the effect, togetherwith the internal insulation 8 and external cooling 9 of the leakjacket, prevents the occurence of a large temperature differential inthe upper regions of the leak jacket thereby avoiding complex stressesbeing created in the leak jacket and its attachment to the roof of thecontainment.

In an alternative construction shown in FIG. 3 the ties 6 comprisestraps welded to the rim of the primary vessel and supported by a flange16 in the roof of the containment. The straps are able to flex toaccommodate any movements due to thermal expansion of the primaryvessel.

In the construction shown in FIG. 4 the ties 6 are composite and eachcomprises a strap 6a extended upwardly by a rod 6b of circular section.The straps are attached to an inverted skirt 15 disposed at a levelbelow the level of the pool surface (designated S) whilst the upper endsof the rods have flanges 16 from which the ties are suspended from theroof structure. The rods are slidably guided by tubes 17 to preventpivoting at their upper ends.

In the construction shown in FIG. 5 the ties 6 each comprise a pair ofrods 6c interconnected end-to-end by a pivot 18. The pairs of rods 6care pivotably connected at their lower ends to an inverted skirt 15 ofthe primary vessel disposed at a level below the surface level 5 of thepool 4 of coolant. The upper ends of the pairs of rods 6c are supportedin the roof structure by flanges 16 and there is a tubular guide 17 foreach of the upper rods to prevent them pivoting at their upper ends.

In an alternative construction each tie (which may be of any of thedescribed alternative kinds) is suspended from the roof structure byjacking means whereby the level of each connection with the primary tankcan be adjusted.

We claim:
 1. A liquid metal cooled nuclear reactor constructioncomprising:a concrete containment vault, a primary vessel housed withinthe vault, said primary vessel having an open top, a nuclear reactorcore submerged in a pool of coolant contained within the primary vessel,a fluid tight leak jacket enveloping the primary vessel and sealed tothe roof of the vault at its rim, the leak jacket containing an inertcover gas for the pool of coolant, and an annular series of tiessuspending the primary vessel from the roof of the vault, said tiesbeing connected to an inverted skirt attached externally to the primaryvessel, the connections being disposed below the level of the surface ofthe pool of coolant.
 2. A liquid metal cooled nuclear reactorconstruction according to claim 1 wherein the ties comprise straps whichare welded to the inverted skirt of the primary vessel and are carriedat their upper ends in the roof structure of the vault by flanges.
 3. Aliquid metal cooled nuclear reactor construction according to claim 1wherein the ties each comprise a pair of rods interconnected end-to-endby a pivot, the pairs of rods being pivotably connected to the invertedskirt of the primary vessel and carried in a non pivotable manner attheir upper ends in the roof structure of the vault.